Homodialysis is a process by which toxin and other molecules, such as urea, are removed from the blood using a semi-permeable filtering membrane. Typically, the patient's blood and an aqueous solution (i.e., dialysate) are pumped in counter-direction flows in and about hollow, semi-permeable fibers. In FIG. 1 a known configuration of a dialyzer is shown. Generally, blood flows in one end of the dialyzer and through hollow semi-porous or semi-permeable fibers toward the blood output side of the dialyzer. Meanwhile, dialysate flows in an opposite direction, with respect to the blood flow, by entering a dialysate inlet and flowing around or about the semi-porous hollow fibers in which the blood is flowing. The dialysate then exits the dialysate outlet. The toxins within the blood are removed from the blood via a combination of diffusion, convection, and osmosis processes while the blood is flowing within the fibers and the dialysate is flowing outside the fibers. Generally, the dialyzer is comprised of a large number of semi-permeable hollow fibers bundled together and placed in a cylindrical jacket as shown. Present day dialysis processes may be classified as: 1) single pass; and 2) sorbent-based. Single pass processes require a continuous supply of gallons of fresh and treated water. The treated water may be purified by for example, reverse osmosis or distillation. The gallons of fresh and treated water are used to create the dialysis fluid, which is discarded after flowing through the dialyzer and collecting the toxins in a single pass through of the dialyzer.
FIG. 2 shows a schematic/diagram of a cross section of a single semi-permeable fiber that may be used in a dialyzer. The blood flows through the hollow lumen within the semi-permeable walls of the fiber. The membrane walls have a thickness, which is the difference of the radius R2 minus the radius R1. The membrane is semi-permeable and the dialysate, as shown, flows in the opposite direction outside of the semi-permeable fiber.
Sorbent dialysis differs from single pass dialysis in that the dialysate is regenerated using a series of chemical powders to remove toxins from the dialysate solution. Typically, spent dialysate from the dialyzer is pumped through the first chemical layer of an enzyme called “urease”. The urease catalyzes the breakdown of urea into ammonia and carbon dioxide. The dialysate will then pass through a second chemical layer, a cation exchange layer (zirconium phosphate) which absorbs ammonia and other positively charged ions and then through a third chemical layer, an anion exchange layer (hydrous zirconium oxide) where anions such as phosphate and fluoride are absorbed. Finally, the dialysate is pumped through a fourth layer of activated carbon where organic metabolites such as creatinine are absorbed. At some point, the filtered dialysate may be passed through a degasser to remove air, carbon dioxide and other gas bubbles that may form or be found in the dialysate.
The capacity of the zirconium phosphate cation exchange layer to absorb ammonia is limited by the number of sites available to bind ammonia. If the zirconium phosphate layer is depleted, ammonia will remain in the dialysate as it is recycled to the dialyzer. In this case the patient may be at risk of ammonia toxicity. Consequently, the filtered dialysate must be periodically tested or monitored for ammonia concentration.
A typical dialysis patient generates an excess of about 24 to about 60 grams of urea per day that must be removed from the blood to avoid uremia. Therefore, what is needed is a sorbent for use in dialysis that has the capacity to remove this quantity of urea in a reasonable time frame. Thus, suitable sorbents should have the capacity to remove approximately 2.5 grams per deciliter of dialysate per hour (gm/dl/hr) from the dialysate.